So far, bridgestrap would only consume Tor's controller events from
c.eventChan as long as the main execution flow was in TestBridgeLines.
Outside of that function, controller events would keep on coming but not
be consumed.  That worked fine for a while because c.eventChan has a
buffer of 100 events but occasionally it would result in the buffer
filling up, which would cause a deadlock because bulb, our controller
library, would block on sending bridgestrap its events.

This patch fixes the issue by also consuming events outside of
TestBridgeLines.  The patch further adds a new Prometheus time series
called pending_events, which captures the number of pending events.  The
time series will help us monitor if this patch got the job done.